1. Field of the Invention
This invention relates in general to the detection and measurement of fluid leaks from containment facilities in the ground. In particular, it pertains to a method for rapidly identifying the occurrence of a leak and for estimating the rate of leakage from the facility.
2. Description of the Related Art
Fluids are stored in tanks or other storage facilities designed to prevent losses and environmental contamination. In particular, liquids are typically contained in enclosed steel and/or concrete facilities or in open structures built over impermeable linings that prevent seepage into the ground. In the case of toxic materials, such as wastes and radioactive substances, it is particularly critical that the containment facility remain leak-proof for a very long time. Accordingly, regulatory requirements have increased over the years to ensure that toxic materials are not released into the environment.
In spite of good storage practices and regulatory mandates, seepage of toxic liquids into the ground is a serious environmental problem and much work has been done to detect underground leaks from storage facilities as promptly as possible after they occur. The early detection of toxic leaks from tanks is particularly important to avoid contamination of aquifers and water streams. This extensive research in the field has produced several successful procedures for locating underground leaks using resistivity-measurement techniques.
For example, U.S. Pat. No. 4,947,470, issued to Darilek (1990), describes a method for detecting and locating leaks in geomembrane liners used to contain liquids by measuring the electric or magnetic field created by a current flowing through a leak. An array of detectors is placed in known locations on one side of the liners and a voltage is impressed across the liner. When a leak occurs, a current flows through the leak and corresponding electric and magnetic fields are created around the leak. Thus, the presence of a leak is sensed by the detector array, and the location of the leak is established geometrically using orthogonal measurements from selected detectors.
In U.S. Pat. No. 5,357,202 (1994), Henderson teaches a method for monitoring a site and locating the presence of leaks from containment vessels by measuring subsurface changes in the conductivity of the soil. Conductivity information is derived from the electrical potential field established in the soil when a predetermined current is imposed between two electrodes. Voltage measurements taken periodically are mapped across the subsoil and converted to corresponding conductivity information. By comparing present conductivity values with a data base known to correspond to normal operating conditions, the presence and location of leaks are detected from changes in the conductivity map of the subsoil.
In U.S. Pat. No. 6,331,778 (2001), Daily et al. teach a similar technique for detecting and locating fluid leaks in liners of containment facilities and in reservoirs, tanks, and the like. Using the well-known mise-a-la-masse (excitation of the mass) technique and tomographic imaging, electrical resistivity data are derived on the basis of voltage measurements between an electrode placed in the fluid and a plurality of electrodes placed in the ground around the periphery of the facility. The leak position is located by determining the coordinates of an electrical current source pole that best fits the measured potentials with the constraints of the known resistivity distribution.
All of these prior-art techniques utilize electrical measurements carried out with electrodes judiciously placed in the soil underlying and surrounding the site of interest. A current is imposed between two electrodes and voltage or magnetic field measurements are taken between various pairs of other electrodes in order to acquire information capable of reflecting changes indicative of a leak of fluid into the soil. As such, these techniques have proven to be useful and reliable in order to detect and locate leaks, but they have not provided information related to the flow rate of a leak. Accordingly, there is still a need for a detection procedure capable of detecting as well as measuring the rate of flow of a leak from a tank or other containment facility.
In particular with reference to radioactive wastes stored in concrete or lined steel tanks, the conventional approach to leak detection has been to sink vertical casings around the perimeter of the tank placed several meters apart from one another. Detection probes are lowered to various depths into each casing to monitor the soil in the vicinity of the casing. If a leak occurs and liquid radioactive material flows within a radius of about one meter from any of the probes, the leak is detected and its location can be estimated on the basis of the position of the casing through which the leak was sensed. While useful when leaks from a tank flow near the probes, this technique is seriously deficient in all other cases and disastrous environmental effects may occur and remain undetected. This invention is directed at providing a detection technique that provides immediate information about the presence of a leak and, at the same time, an estimate of the flow rate of the leak.